Neutron scintillation detector



United States Patent The present invitation is concerned with senior c'qunfter's" amt more particularly with compositions of matter suitable'for' employment as scintillation niediuins' insciritil ladenneutroncountersor detectors; H

Theme vvide'l'y'us'ed generictype'ofcounterfM atomic: aaunucle'ar particles is'thjat bas'ed upohfa comb as of we electrodes in a gas; Thep'a'ssage are charged atofr'i'ic particle through the gas iv ill result in ionization" or the production of ion pairs; The positiveibns' will move totli'e eg'ative electrode and the corresponding negative ionsto thepositive' electrode; resulting in a'p'ulse in tlie electri cal system co'riiiect'ed vvith the-electrodes; Dep nding upon tlimethodin'vvhicli they are operated; these counters ar' usually feir'ned an ionization chamber,- pro er counter, or a Geiger counter; Since these couh'tersI enl-y respond to charged particles,- they causes be" used as neutron counters Without area-meanest- I The modification normally u'se d is inc-introduction are compoundof a substance such as boron or lithiuin v'vhic'li" reacts With neutrons to form charged pa ticles: The iso tope'B' (which comprises one-firth of natura-uyece boron) reacts with thermal neutronsto form charged alpha particles accordingto the following .reactiorr:-

The boronis usually introduced into the chamber as e'rgas; 40 for example BF3,- bllli'il; may also be placed in the chamberasa-coating on the inner surface of th'e chamber in the formpf some suitable compound. The B responds primarily to slow neutrons or thermal neutrons so' that' an additional modification must be' made in'the counter it it is to be used for countingfast neutrons. Theusualmodification is the surrounding of the counter with anhydrognous material, coating or layer, so that the neutrons passing throughthe layer are slowed down to thermal energies. Now,- while the reaction of B with neutrons has 1 a large cross section, counters employing such a method have some very obvious disadvantages, perhaps the chief of which is the low efiiciency of the counter. The method of determining the efficiency of BF; neutrefi-coum trs is described in Electronand Nuclear Coun'ters Theory and Use-f-Korif, Van Nostrand, 1946;; page 5 2. The makimum efficiency that may be expected for BEafilled neutron counters is of the order of 10% and the maximum efiiciency for boron-lined counters is of the order of 5 I I I it is an object of the present invention to provide a neutron counting device having an efliciency approaching It is an additional object of the present invention to provide compositions of matter suitable fo'r'use as" the scintillator in a scintillation counter rer'aenaens. I I

We have discovered that a neutron detector, which: is approximately 100% eflicient, has a very slidrt'respbase time; and is relatively insensitive to gamma rays. can be constructed by employing novel compositions are Hereinafter described as the scintillation filedlliiji" of fs cinti-lla'tor iii a scintillatioii counter. The novel cdiii'-" 2,755,253 Patented July- 17, 1956- positions which -wehave discovered have three components; aradiator, aphosphor; and --a solvent; The radi= atoris theconstituent of; our solution which reacts: with neutrons to produce charged particles; The charged particles thus-'produced-then react with the phosphor and solvent'to produce scintilla-tions which are countedbyia photomultiplier tube orbthersimilarphotosensitivedevice. Theexact mechanism by which scintillations are producied'ha'sj'not" been finally determined, but it is be lieved that tlie" e'riergy fromtli'cliargedparticles is first absorbed by the" solvent molecules, and their at least partially transferred to the phosphor molecules; wliichin' turn emit the energ in the form of light waves. In any event the clibice ofa solventcanno't bc'rn'ade solelyupon solubility chai'ac ristics.

' which are employed our scintillating mediums arearornatic or aralkyl compositions containing a" large proportion of double bonds; Aliphati'c b orate compounds stressed as the radiators and alkyl ar'yl; andaralkyl liquid organic compositions are useful as the isfi a I "ile'various fluorescent materials and solvents areknown; the known materials cannot be combined indiscnnnnately s obtain a product: useful as the detectingmedi '11 'a liduid'xieut'rdn'scintillation counter. There are ej I optica ;physiea1; ciienncarand elcctrical properties Whiblia're of impatiencenutter-mining the usefulness of the final pr'iiiii'lct. Tli'rniiifure of phosphor, radiator and solvent must be clear and should be colorless and have a high transmission factor for visible as Well as for ultraviolet light. The composition should be fairly-- dense and be comprised of atomicconstituents having low atomic numbers. With respect to the chemical propertiesof the product, the three components preferably form a -true solution and this solution should be stable over a considerable range of temperature. For example, the neutrondetector medium in a scintillation counter is often cooled to -a low temperatuie of the order'of 20 C. during the cooling of thephotomultipliehtube in order to-natiuce spurious and background counts and a scintillation ni'e dium in Which-one-of the components will precipitate out response isimport-ant and discrimination between alphas and gammas is of the utmost importance. The 'efiicicncy of alpha detection must be consideredand the spectrum' of ithelight emitted is also important, since it should be oi a suitable spectral'range to couple Withthe phototube or other light-sensitive counting'd'viee. The final characti is'tics which determine the suitability of a mixture ofphor, radiator and solvent for a neutron detector mediiiiii'cannot be det'erin ined theoretically from a know]- edgof the individual characteristics of the components running the mixture but dependgrea-tly upon the interteacher the components of the mixture not only with respect to the substance used but also with respect to the tidlYS.

I II liosphorsvvhichaf employed in the compositions of the preset Invention are in general complex aromatic orgahi compositiohs'liai iiig a large proportion of double beads to the molecule. general formula iii w'hi c'h Rr'e'pres'ents a phenyl ring; a heterocyc'lic ring cent mng at'leas't one-double bond or a conjugated ali phat I sys'e" I WheieR represents a' plffiyl 'ringthe rin'g be joined Preferable phosphors have the havingnot more than eight earbonatoms."

to the outer phenyl rings by single bonds or may be fused. Examples of phosphors in which R represents a phenyl ring include terphenyl, anthracene, and phenanthrene. R represents a conjugated aliphatic linkage in such compounds as diphenylbutadiene, diphenylhexatriene, and diphenyloctatetraene. An example in which R represents a heterocyclic ring is 2,5-diphenyloxazole. The phosphors which are preferable in particular compositions include 2,5-diphenyloxazole, terphenyl and diphenylhexatriene. Phosphors which may be used in particular combinations include the following:

Fluorene Naphthalene Carbazole (diphenyleneimine) a-Napthol Phenanthrene fl-Napthol Anthracene a-Napthylamine Diphenyl Phenol m-Diphenylbenzene (terphenyl) Trans-stilbene Diphenylbenzidine Triphenylamine Diphenylbutadiene Dihydrocollidine Diphenylhexatriene Carotene Diphenyloctatetraene Diphenylacetylenc Fluoranthene Melaniline The solvents which are employed are organic compositions which are normally liquid at room temperature. The preferred solvents are aralkyl compositions having a single phenyl ring with an attached aliphatic group or groups, such as phenylcyclohexane and xylene. The commercial mixture of xylene isomers or the pure isomers may be used. Other organic solvents may be used in particular combinations. Such solvents include:

Aniline Hexane Benzene Mesitylene Benzyl alcohol Napthalene monobromide Benzyl ether Pyridine a-ChlOI'Ol'lflPihfilCl'lB Quinoline Cyclohexane Styrene 1,4-dioxane Toluene n-Heptane The radiators which are employed in our composition are aliphatic borates, in which the aliphatic groups are straight chain groups having not more than four carbon atoms. Trimethyl borate has been found to be the preferable boron composition because of the high boron per molecule atomic ratio. Triethyl borate, tripropyl borate, tributyl borate or aliphatic borates having mixed aliphatic groups may be used. While the boron content of the aliphatic borate may be natural boron (B, 18.8%, B 81.2%), the boron enriched in B is preferred.

The solvent, for example xylene or phenylcyclohexane, is normally combined with the radiator, for example trimethyl borate, in a proportion of about one to one by volume. This ratio may be varied from about 25% solvent-75% radiator, to 75% solvent-25% radiator. This variation, however, may result in reduced pulse height and decreased temperature solubility range if the radiator is increased in volume at the expense of the solvent, or in reduced efficiency if the solvent is increased at the expense of the radiator. The scintillating material is usually added to the mixture of solvent and radiator in proportions of about 2 to 8 grams of scintillating material per liter of mixed radiator and solvent. Mixtures of two scintillating materials may be used and often a minor amount of a second scintillating material may be added to the composition to shift the spectrum of the emitted light to one more suitable for the particular photomultiplier tube used. For example, when the photosensitive device to be used is the 5819 photomultiplier tube and the main scintillator is 2,5-diphenyloxazole or terphenyl, a few milligrams per liter of diphenylhexatriene may be added to the mixture to shift the spectral response. Between 2 and 20 milligrams per liter of diphenylhexatriene is usually sufficient to shift the response to an imtil proved spectral region for detection by a 5819 tube. A particularly suitable liquid scintillator composition which may be used at a temperature as low as -10 C. is as follows.

(a) Equal volumes of trimethyl borate and phenylcyclohexane containing 4 grams per liter of para-diphenylbenzene (terphenyl) and 8 milligrams per liter of diphenylhexatriene. A second composition which gives larger pulse heights than does the first one and whose solubility characteristics are such that it can be used at a temperature as low as 20 C. is as follows.

(b) Equal volumes of trimethyl borate and phenylcyclohexane containing 4 grams per liter of 2,5-diphenyloxazole and 16 milligrams per liter of diphenylhexatriene. The spectral response of these two compositions is particularly suitable when they are used with a 5819 photomultiplier tube as the detecting device, and the diphenylhexatriene may be eliminated from the compositions if they are to be used with a scintillation detecting device which responds to a lower spectral range. The scintillation medium is placed in a suitable cell made of material or materials having low atomic numbers such as quartz or polystyrene. The cell is light-tight except for a transparent window which communicates with the photosensitive device. The light-tight coating may be made by placing the cell in an aluminum container, or coating the surface with aluminum foil or magnesia. The window of the cell is placed in conjunction with a photosensitive device such as a phototube or photomultiplier tube. Photomultiplier tubes such as the IP21 or 5819 are particularly suitable photosensitive devices. These are highvacuum phototubes containing a plurality of dynodes. The photocurrent produced at the cathode is multiplied many times by secondary emission occurring at successive dynodes. The equivalent noise input of such tubes is low at room temperature, but can be still further decreased by cooling such tubes to below room temperature, for example 10 C. The window of the tube is usually connected with the window of the cell by a light pipe made of an acrylic resin such as Lucite. The pulses from the photomultiplier tube are counted with conventional electronic counting devices. Measurements of counting rates obtained with the above-described solutions (a) and (b) in counter cells approximately 2 inches in diameter and 1 inch long coupled with the 5819 photomultiplier tube connected to a suitable counter circuit have indicated that the efiiciency of counting of the neutrons in the scattered thermal neutron flux of a heavy water reactor was approximately 100%.

The mean life for capture of neutrons in the energy region in which the 1/v law for boron is operative is approximately 0.42 microsecond if enriched boron is used in solutions (a) and (b) above, and approximately 2.2 microseconds if normal boron is used in the above compositions.

The gamma discrimination for counters containing solutions (a) and (b) above and operated with a counting mechanism biased to give the optimum counter response necessary for detection of boron disintegration pulses was found to be good. Both experiment and calculation indicated that the response of the meter used as a slow neutron counter to gamma rays (produced by the Compton effect) was approximately 2% per gram per square centimeter formula in which R is a member of the group consisting of a phenyl ring, a heterocyclic ring containing at least one double bond, and a conjugated aliphatic chain having not more than 8 carbon atoms.

2. The composition of claim 1 in which the aliphatic borate compound is trimethyl borate.

3. The composition of claim 1 in which the aliphatic borate compound is triethyl borate.

4. The composition of claim 1 in which the phosphor is terphenyl.

5. The composition of claim 1 in which the phosphor is 2,5-diphenyloxazole.

6. The composition of claim 1 in which the solvent is phenylcyclohexane.

7. The composition of claim 1 in which the solvent is xylene.

8. A neutron detection composition capable of producing scintillations, comprising a mixture of 25-75 weight per cent phenylcyclohexane and trimethyl borate, containing 2-8 grams per liter of said mixture of 2,'5-diphenyloxazole and 2-16 milligrams per liter diphenylhexatriene.

9. The composition of claim 8 in which the boron of the trimethyl borate is enriched in the boron isotope B 10. A neutron detection composition capable of producing scintillations, comprising a mixture of 25-75 weight per cent xylene and trimethyl borate, containing 2-8 grams per liter terphenyl and 2-16 milligrams per liter diphenylhexatriene.

11. A neutron detection composition capable of producing scintillations, comprising a mixture of weight per cent phenylcyclohexane and 50 weight per cent trimethyl borate, said mixture containing 4 grams per liter of 2,5- diphenyloxazole and 16 milligrams per liter of diphenylhexatriene.

12. A neutron detection composition capable of producing scintillations, comprising a mixture of approximately 50 Weight per cent 2,5-diphenyloxazole and 50 weight per cent trimethyl borate, containing 4 grams per liter of said mixture of terphenyl and 8 milligrams per liter of said mixture of diphenylhexatriene.

References Cited in the file of this patent Review of Scientific Instruments, July 1951, vol. 22, No 7, page 543. 

1. A NEUTRON DETECTION COMPOSITION CAPABLE OF PRODUCING SCINTILLATIONS, COMPRISING A MIXTURE OF 25-75 WEIGHT PER CENT ARALKYL ORGANIC SOLVENT OF THE GROUP CONSISTING OF XYLENE AND PHENYLCYCLOHEXANE AND AN ALIPHATIC BORATE IN WHICH THE ALIPHATIC GROUPS ARE STRAIGHT CHAIN GROUPS HAVING NOT MORE THAN 4 CARBON ATOMS, SAID MIXTURE CONTAINING 2-8 GRAMS PER LITER OF AN ORGANIC PHOSPHOR HAVING THE FORMULA 